1. Field of the Invention
This invention relates to a switching power supply which has a boost converter, and is capable of limiting an inrush current flowing into a smoothing capacitor.
2. Description of the Related Art
In general, a switching power supply incorporating a boost converter has a smoothing capacitor provided therein for smoothing an output voltage from the boost converter. Accordingly, a large inrush current flows into the smoothing converter via the boost converter at the initial stage of power-on. Therefore, this type of switching power supply generally has a power thermistor or a resistor arranged at some location between an AC input and the smoothing capacitor, so as to supply an AC voltage via the power thermistor or the like at the initial stage of power-on, thereby limiting the inrush current to a predetermined value.
A boost power supply 41 shown in FIG. 6 is among switching power supplies of the above-mentioned type as the related art. The power supply 41 includes a fuse 3 for protection against overcurrent, a rectifying diode stack 4, a power thermistor 6 connected between the fuse 3 and the diode stack 4, a boost converter 5, and a smoothing capacitor 7. The boost converter 5 is comprised of a choke coil 11 for voltage boost, a switch 12 formed by a switching element such as an FET, and a diode 13.
In the power supply 41, at the initial stage of power-on, an AC voltage VAC is input from an AC power source 2 to the diode stack 4 via the fuse 3 and the power thermistor 6. At this time, the internal temperature of the power thermistor 6 is approximately equal to the ambient temperature, and hence the power thermistor 6 has a larger resistance value (e.g. 10 .OMEGA.) than a resistance value (e.g. 1 .OMEGA.) it has when heated. Therefore, if an AC voltage VAC of 100 V is input at the initial stage of power-on, with its phase angle of 90 degrees, i.e. at its peak, a peak current IP of the inrush current is limited to approximately 14.1 A since the peak voltage of the AC voltage VAC is approximately 141 V.
On the other hand, when the capacitor 7 is charged to a predetermined voltage, the switch 12 within the boost converter 5 is controlled by a switching control circuit, not shown. At this time, the boost converter 5 outputs excitation energy of the choke coil 11 via the diode 13 to thereby output a voltage which is higher than a pulsating voltage V1 rectified by the diode stack 4. In this case, the input current IIN dependent on the AC voltage VAC flows through the power thermistor 6, whereby the power thermistor 6 heats itself. As a result, the power thermistor 6 has the resistance value thereof reduced to a smaller value as its temperature becomes higher. Further, when the resistance value has been reduced, the amount of heat generated by the power thermistor 6 becomes small. Thus, in a normal state after power-on of the power supply 41, the parameters consisting of the temperature of the power thermistor 6 increased as it heats itself and the resistance value of the same are stabilized in a predetermined condition, so that in the above example, the resistance value of the power thermistor 6 becomes equal to approximately 1 .OMEGA.. In this state, power loss occurring when the input current IIN flows through the power thermistor 6 has been made small, so that the inrush current is limited while attaining the enhancement of the conversion efficiency of the device.
However, the power supply 41 suffers from the following problems: Firstly, there still remains a resistance of approximately 1 .OMEGA. in the above example. For this reason, in constructing a switching power supply having a large output, power loss due to the remaining resistance is not negligible, and hence improvement of the conversion efficiency is still desired.
Secondly, another problem occurs when the power is turned on again after restoration of input from momentary interruption. More specifically, in the above example, e.g. when the power supply 41 is continuously operated under an ambient temperature of 40.degree. C., the power thermistor 6 is stable with its resistance value held at approximately 1 .OMEGA.. In this state, even if input of the AC voltage VAC is interrupted for a short time (dozens to hundreds of milliseconds, which will cause the charging voltage of the capacitor 7 to drop to a voltage value close to 0 V) due to a power failure or the like, the AC voltage VAC starts to be input again before the temperature of the power thermistor 6 is lowered sufficiently. Assuming that the resistance value of the power thermistor is 1 .OMEGA. when the AC voltage is input again, since the peak voltage of the AC voltage VAC is approximately 141 V, the peak current IP of the inrush current is approximately 141 A. A flow of such a large peak current IP not only causes blowing or deterioration of the fuse 3, momentary interruption of the commercial power line, and tripping of a power breaker for household or commercial use, but also it exceeds rated surge current of various electronic components, such as the diode stack 4, arranged in a conductive line for the peak current IP, thereby causing breakage or degradation of the electronic components. This problem inevitably occurs so long as the power thermistor 6 is employed as inrush current-limiting means. Therefore, in general, as a countermeasure against the inrush current after restoration of input from momentary interruption, there is employed a method in which a resistor having a resistance value of several ohms is constantly connected to the conductive line of the AC voltage VAC, thereby limit the peak current IP of the inrush current below a predetermined value or a method of selecting for use electronic components which can withstand the peak current IP which will flow after restoration of input from momentary interruption. However, when the former countermeasure is taken, the constantly connected resistor causes constant power loss, which results in a lowered conversion efficiency of the power supply. On the other hand, the problem with the latter countermeasure is that selection of a fuse 3 having a large rated current for use prevents the fuse 3 from properly blowing and that the size of each electronic component is inevitably increased.
It should be noted that if a resistor is used as inrush current-limiting means in place of the power thermistor 6, large inrush current does not occur after restoration of input from momentary interruption, but the method is basically similar to the above former countermeasure in that power loss is caused by the resistor. Therefore, although the method is somewhat effective in a power supply of a type having a relatively small steady-state current flowing therein, in a power supply of a type having a relatively large steady-state current (e.g. 1 A) flowing therein, large power loss caused by the resistor (e.g. power loss of 10 W when the resistance value is 10 .OMEGA.) occurs constantly, which considerably lowers the conversion efficiency of the device.
A power supply 51 shown in FIG. 7 is among switching power supplies as the related art which can attain high conversion efficiency even with a smaller steady-state current flowing therein. In the power supply 51, at the initial stage of power-on, a thyristor 52 is controlled to be in an OFF state, which allows a power thermistor 6 to limit the inrush current. Then, when the charging voltage of a capacitor 7 reaches a predetermined voltage, an activation circuit 61 activates switching control circuits 62 and 63. As a result, a boost converter 5 has its switching operation controlled by the switching control circuit 62 to boost an AC voltage VAC for charging the capacitor 7. At the same time, a switch 9 formed by a switching element such as an FET or the like is controlled by the switching control circuit 63 to switch the charging voltage of the capacitor 7. Consequently, a current flows through a primary winding 8a of a transformer 8, whereby a voltage induced in a secondary winding 8b is generated as an output voltage VO. In this case, a voltage is also induced in an auxiliary winding 8c, and this induced voltage is rectified and smoothed by a diode 53 and a capacitor 54. Then, the rectified and smoothed DC voltage is input to the gate of the thyristor 52, whereby the thyristor 52 is controlled to be in an ON state.
As a result, the steady-state current passes through a diode stack 4 and the thyristor 52 to be input to the boost converter 5. In this case, the voltage between the anode and cathode of the thyristor 52 is approximately 1.1 V to 1.5 V, and hence, if the steady-state current is 1 A, power loss caused by the thyristor 52 in the steady state amounts to approximately 1.1 W to 1.5 W. This power loss is larger than that caused by the power thermistor 6 of the power supply 41. However, if the steady-state current is 5 A, for instance, the power loss caused by the power thermistor 6 is 25 W (5 A.multidot.5 A.multidot.1 .OMEGA.), whereas the power loss caused by the thyristor 52 is 5.5 W to 7.5 W (5 A.multidot.1.1 V to 1.5 V), which means that the latter is more excellent in conversion efficiency than the former. It should be noted that a switching element such as a triac, an FET, a transistor, or the like can be used in place of the thyristor 52. Further, it is possible to use a resistor in place of the power thermistor 6.
A power supply 71 shown in FIG. 8 is also among the switching power supplies as the related art which can attain high conversion efficiency. This power supply 71 is similar to the power supply 51 in that a boost converter 5 and a switch 9 are controlled by switching control circuits 62 and 63 (not shown). In a steady state of the power supply 71, thyristors 52 and 72 are each controlled to be in an ON state by a DC voltage generated by a diode 53 and a capacitor 54, whereby an AC voltage VAC brings the thyristors 52 and 72 into conduction in place of a diode stack 4 to be input to the boost converter 5. As a result, the forward voltage (e.g. 0.98 V) of one diode incorporated in the diode stack 4 and a voltage drop by a power thermistor 6 is limited to the forward voltage of the thyristor 52 or 72. In this case, the power supply 71 is distinguished from the power supply 41, in which when the steady-state current is 1 A, power loss of 1 W is caused by the power thermistor 6, in that power loss caused by the thyristor 52 or 72 is relatively reduced to 0.12 W to 0.52 W ((1.1 V to 1.5 V).multidot.1 A-(0.98 V 1 A)). Thus, the power supply 71 has a further improved conversion efficiency.
However, the power supply 71 suffers from the following problems:
Firstly, the power supply 71 requires the use of the two thyristors 52 and 72, a transformer 8 using an auxiliary winding 8c, the diode 53, and the capacitor 54, which makes its circuitry complicated and hence increases manufacturing costs.
Secondly, during momentary interruption of input, even after the charging voltage of the capacitor 7 starts to drop, the switch 9 continues its switching operation until the charging voltage is sufficiently lowered. Therefore, the inrush current is allowed to flow into the capacitor 7 via the thyristors 52 and 72 when the input is restored from momentary interruption before the charging voltage is sufficiently lowered, and hence it is impossible to adequately limit the inrush current flowing after the restoration of the input.
Thirdly, after the restoration of the input from momentary interruption, since the inrush current is not limited by the power thermistor 6, it sometimes happens that an extremely large peak current IP (e.g. from dozens to hundreds of amperes) flows through the thyristor 52 or 72. As a countermeasure to this problem, it is required to select thyristors each having a rated current value large enough to withstand the surge current. In this case, the use of FETs or transistors would make it possible to decrease power loss in the ON state, but the permissible surge current of these elements is generally small. Therefore, actually, the use of thyristors or triacs having large permissible surge currents as well as slightly larger ON-state voltages (i.e. allowing large power losses as well) is indispensable for preventing the power supply 71 from breakdown. Consequently, the power supply 71 suffers from a problem that the conversion efficiency thereof is actually lowered due to the power loss caused by the thyristors 52 and 72.